Wales were on Saturday night coming under mounting pressure to explain why medics allowed George North to carry on playing against England after the winger appeared to be knocked unconscious twice during Friday's night's RBS Six Nations opener.

World Rugby, the sport's governing body, issued a statement requesting a 'full report' from the WRU to explain the circumstances surrounding the decision to allow the 22-year-old to remain on the field despite television pictures clearly showing him slump motionless to the ground after suffering a 61st-minute head injury.

North had earlier passed a Head Injury Assessment (HIA) after being accidentally kicked in the face by England lock Dave Attwood in the first half, when he also appeared to be out cold temporarily.

George North (left) is taken off the field by Wales team doctor Geoff Davies before later returning

North (bottom) is inadvertently kicked in the face by England second row Dave Attwood on Friday

The Wales wing appeared to be knocked out twice during his side's 21-16 Six Nations loss against England

World Rugby guidelines state any player who is even suspected of losing consciousness should immediately be removed from play to avoid the risk of the potentially fatal 'Second Impact Syndrome'.

Their statement said: 'World Rugby has requested a full report from the Welsh Rugby Union following an apparent head impact sustained by Wales wing George North. Having undertaken an initial review of the broadcast footage, World Rugby is seeking to establish whether all concussion management protocols were appropriately followed.'

North, who missed last autumn's win over South Africa with concussion, was left dazed following the initial incident and, while Wales coach Warren Gatland defended his medical team, the decision to let him play on has drawn stinging criticism.

The WRU insisted on Saturday night that correct procedures were followed, although they admitted the second incident had not been picked up by medical staff at ground level, despite TV footage being available to them.

A WRU statement read: 'While George is currently symptom free, retrospective video review of the second incident identified the mechanism of injury which was previously unsighted on the field of play. This review has warranted the medical team to manage the player as concussed although the player currently has no signs and symptoms. He will now undertake a graduated return-to-play protocol with multiple follow-up cognitive and physical tests.'

North's (right) second apparent case of concussion came from a tackle by team-mate Richard Hibbard (left)

North lays poleaxed on the Millennium Stadium turf after the nasty-looking collision on Friday night

Gatland said immediately after the match that he had not seen the second incident, although his attempt to trivialise the concussion suffered by prop Samson Lee in the same game by saying: 'He should be OK, he's got a pretty thick head,' suggested a worryingly cavalier attitude to the issue.

'The medical team definitely wouldn't have allowed George to go back unless they were 100 per cent confident that he was OK,' Gatland said. 'He seems fine at the moment.'

The Head Injury Assessment allows team doctors 10 minutes to assess a player with suspected concussion off the field. But in the case of a player losing consciousness, or being suspected of losing consciousness, protocols state they should be removed immediately from the field without undergoing an HIA.

'Rugby has come a long way in concussion management in the past couple of years, but events in the Millennium Stadium show there is still a lot of work to do,' said leading head injury expert and World Rugby advisor Professor Willie Stewart.

North (right) and his captain Sam Warburton look on during Wales' defeat to England

The 22-year-old (left), missing a tackle on Jonathan Joseph, saw his game suffer after the first head injury

'For a concussed player, the consequences of being left on the field are considerable and include risk of further and more significant injury, perhaps with prolonged recovery running to weeks or months, or worse.

'However, for those making the decision to leave a player on, there are no consequences. No fines. No points deductions. No licences lost. This must change if real progress in player welfare is to be made.'

Concussion has become an increasingly difficult issue for rugby and other professional contact sports to grapple with after it emerged that former American footballers exposed to repeated head injuries were 19 times more likely to suffer serious long-term neurological problems than the wider population.

The decision to allow North to play on drew huge criticism on social media. Former England captain Lewis Moody, who last year admitted he used to treat concussion as 'a joke' during his playing career, tweeted: 'Why was George North not taken off terrible decision by the medics. Out cold.'

Former Scotland international Rory Lamont tweeted: 'George North knocked out twice in one game and still on the pitch. How is this still happening?'

North puts his hands on his knees and looks on during Wales' loss to England at the Millennium Stadium

The Wales wing (right) tries to tackle England's Anthony Watson during the Six Nations clash

BBC commentator Brian Moore also raised serious concerns about the decision to allow North to play on.

Last year's RFU injury audit found that concussion was again the most common injury suffered by professional players in the Aviva Premiership. The audit for the 2013/14 season will be published this Wednesday.

World Rugby intend to employ independent matchday doctors at this year's World Cup whose job will be to alert team medics to suspected concussions in a bid to avoid a repeat of the North incident.

'In an ideal world we'd have independent doctors at every single professional rugby match but it always comes back to cost,' said one well-placed source.

North will now face a graduated six-day process before he can be passed fit, with his availability to face Scotland in Edinburgh next Sunday in serious doubt.